This invention relates to a brazing flux. More particularly, this invention relates to a dip brazing flux for brazing aluminum alloy surfaces.
The joining together of metal surfaces is accomplished either by melting the surfaces to cause a fusion therebetween, which is normally referred to as a welding type joint, or by providing a lower melting temperature alloy between the surfaces to be joined together which melts and wets the surfaces of the metals without melting the metals themselves. Depending upon the joining temperature, such operations are usually referred to either as brazing or soldering. In all of the foregoing joining operations, the presence of metal oxides on the surfaces of the metals to be joined can interfere with a proper joining together of the surfaces.
It has therefore become the accustomed practice in the various joining arts to use materials known as fluxes which will have a solvent effect on such oxides to provide clean surfaces to insure proper wetting of the solid metal surface by the liquid metal. Such fluxes normally comprise metal halides, particularly alkali metal halides such as sodium or potassium chloride or fluoride.
The flux or fluxing agent may be applied in a number of ways as by dusting a powdered mixture of the flux ingredients onto the surface prior to the joining operations, applying a liquid paste of the material or, in the case of dip brazing, by providing a molten mixture of the flux into which the parts to be brazed are dipped. The molten batch also then provides the necessary heat to melt the alloy material which is used as the brazing metal between the surfaces to be joined together.
In the joining together of aluminum alloy materials, a flux must be used to remove aluminum oxide which would otherwise interfere with the proper joining together of the parts. Sometimes calcium chloride is used in such fluxes, however not without problems, particularly when used in conjunction with NaF or AlF.sub.3. For example, it has been found that the presence of fluoride ions and calcium ions in some flux compositions may result in the formation of an insoluble compound, calcium fluoride, when the flux is used in a molten bath for brazing purposes. The calcium fluoride formation is detrimental in that it acts to deplete the flux of these constituents prematurely, often resulting in the flux having a useful life of not more than one day. Such a short life makes the use of this type of flux quite impractical from an economic viewpoint.
Fluxes employing a metal fluoride and chlorides of sodium, potassium and calcium are known for welding magnesium. Such a use is disclosed in U.S. Pat. No. 2,396,604. However, as well as using the flux for welding magnesium, the patent discloses that the flux is applied by dusting or a paste mixture in water, alcohol or oil, and preferably uses sodium fluoride therein. Thus, the stability of the flux would not be a problem. In welding, the filler metal has essentially the same composition as the components to be joined and an appreciable amount of the parent metal is melted during the joining process. Brazing is considered to be a different joining process in that the filler metal is typically an alloy which melts at lower temperatures than the parent metal and usually melting of the parent metal does not occur.
Since dip brazing fluxes are preselected to provide an operating temperature which will be above the melting point of the brazing alloy, yet below the melting point of the surfaces to be joined, only particular combinations and ratios of metal salts, such as metal halides may be used, which will provide such a temperature range.